Cemented zirconium boride material having a protective chromium containing coating



- cess of chromium.

'CEMENTED ZIRCONIUMV BORIDE MATERIAL HAVING ,A- PROTECTIVE" CHROMIUM' CON- AI I A N Frank W. Glaser, NewYork, N.Y., assignor, by mesne assignments, to Borolite Corporation, Pittsburgh, Pa., a corporation of. Delaware N w g- Application June 22, 1953 Serial No.3 63,3' 76 2 Claims- (Cl. 117-118) This application is a continuation-in-part of my application SerialNo. 170,240, filed June 24, 1950.

This invention relates to shaped bodies of great strength at elevated temperatures which are resistant to corrosion Inlmy co-pending application Serial No. 363,043, now i abandoned, filed June 22, 1953, as a'continuation-in-p'art oflmy said application "Serial No. 170,240, now aban doned, there is'disclosed and claimed a new hot strength.

. corrosion-resisting structural material consisting ofce mented zirconium diboride I particles bonded by 2-7% "of carbon-containing boron. f 1- The'pre'sent invention is based on the discovery that shaped articles of great hot-strength and corrosion-re sistanc'e atelevated temperatures'for applications such as gas turbines, jetengines, and other applications of the I foregoing YPe When made out of cemented particles 6f zirconium diboride ZrB which are bonded with about 2' to 7% boron containing in solid solution about 4 to 33 atomic percent carbon, although as such are superior to other known materials of this type, heretofore used in or proposed for such applications, may be given a much greater life in hot corroding atmospheres by providing the exterior exposed surface layer of such body, with a surface stratum consisting of chromium oxide, chromium I, and boron, the chromium and boron being present in the s'urface'stratu'm as dichromium b'orideCr B with an ex- 7 (Throughout this specification and claims, all proportions are given by weight unless other{ wise specifically stated.)

In other'words, the present invention is based on the discovery 7 that shaped bodies formed of particles of ZrB that are bonded with 2to 7 boron containing in solid solution about 4 to 33 atomic percent carbon, may

' be; given considerably higher life in corrosive atmospheres if the exposedexterior of such=body is provided with'an" integral protective surface layer which is substantially;

' free 'of'ZrB and contains essentially chromium oxide, chromium and boron, with the boron present principally as a part of a new compound dichro-mium boride Cr B.'

'A distinct feature of the invention is 'a shaped'body having an exposed surface layer of particles" of zirc'onium diboride ZrB bonded with 2' to 7 percent boron containing in solid solution about 4 to 33 atomic percent car- 7 bon, and having a surface stratum that is exposedfto the 7 hot corroding atmospheres, and which consists'of chromium oxide, dichromium boride Cr B, and an excess of chromium, and which is substantially frjeeof zirconium and zirconium diboride.

2,936,250. Patented May .10, 1960 Breparation of zirconium diboride powder In one form of a satisfactory process of the invenof the invention having the desired highresis'tanceto oxidation, corrosion and heat shock, it is essential that they shall be of a minimum purity of between 96.5 to 99% plus, i.e., that 96.5 to 99% plus of the cemented body shall consist of zirconium, boron and carbon only as ZrB with 2 to 7% carbon containing boron. Q It is important that the iron impurities be less than about 0.2% and preferably less than about 0.1%. Itis also important to suppress and eliminate impurities of silicon carbide and other silicon impurities. I I

The zirconium and the zirconium hydride contains sometimes hafniumas an impurity, and such impurity.

the cemented zirconium diboride ZrB which may also contain in solid solution minor impurities of other metals of the fourth group of the periodic system. These impurities may be present in amounts up to about 5.% by weight without impairing the properties of the cemented zirconium diboride ZrB material to any appreciable extent with respect to oxidation, corrosion-resistance and shock resistance.

The mixture of the starting powder ingredients is compacted to reduce the powder volume, using dies and press sures of about to 4 t.s.i., this pressure range having no critical significance. The powder mixture compacts; are then placed into a graphite crucible whichhas a .cover equipped with three stacks. One cover stack serves'for the introduction of hydrogen into the crucible and an; other stack for burning the hydrogenleaving the crucible. The third stack serves for optical control of; the tem-Q perature in the crucible. The cruciblewith the powder ingredient'mixture contents is placed in a high frequency induction furnace, the crucible being provided with an insulating coating of lamp-black powder to reduce heat losses while being heated.

Before starting the heating, the interior of the crucible is purged with hydrogen and after purging, the hydrogen is ignited and the high frequency heating started while maintaining in the interior of the crucible, an atmosphere, of hydrogen. 7 heating with an alternating current of about 1000 to 3000 cycles per second. By way of example, with a powder mixture load of about 50 to pounds held in a graphite crucible of 18 inches inside diameter, 4 feet height'and 2 inches wall thickness, good results are obtainedwith an induction heater equipment capable of 60 kilowatt heating energy. Good results are obtained by heating with 40 to 50 kilowatts during the first 60 to minutes of the heating cycle until the load is brought to a. tern perature of about 700 to 900 C. When thist'ernpera ture is reached, the heat input is lowered since 'a' rela tively violent exothermic reaction takes place in the pow der mixture as the zirconium diboride ZrB is being formed out of the powder ingredients at a temperature" between about 1000 and 1200 C. Because of'the heat developed at this stage of the process, by the exothermii: reaction of the powder ingredients, the heat Good results are obtained by'induction'.

During the initial heating cycle at which the temperature of the powder mixture in the crucible is increased to about 900 C., there will take place a gradual decomposition of the zirconium hydride yielding continuously atomic hydrogen thereby providing the powder charge with a very effective additional protection. If zirconium powder in a form other than hydride is used in the starting powder mixture, the ultimate zirconium diboride obtained will be of somewhat lower purity.

Upon completion of the exothermic reaction, the full high frequency heating power of the induction heating equipment is applied to the crucible and there will be observed a relatively slow further rise of the temperature from about 1200" C. The slow temperature rise is due to the fact that the powder load has been greatly decreased by the formation of the zirconium diboride ZrB which has considerably higher heat conductivity than the original powder mixture in the crucible. It is desirable to have an additional heating cycle of about one hour with about 60 kilowatt heating energy in which the load is heated from 1200 to 1800 C., in order to eliminate impurities from the zirconium diboride ZrB that have been introduced by the use of impure boron in the original boron powder mixture. The principal boron impurity of the original powder consists of magnesium oxide which is reduced at temperatures about 1400 C. By heating the load to about 1800 C. with carbon originally introduced in the powder mixture or by the atmosphere of carbon oxide CO generated by the heated carbon crucible, the magnesium oxide will be reduced to magnesium metal which at temperatures between 1600 and 1800 C. will volatilize and leave or be expelled from the crucible, yielding a body of high purity zirconium diboride ZrB with the desired excess of 2 to 7% of carbon-containing boron.

The treated powder body should be maintained in the crucible at about 1800 C. for a period sufficient to eliminate all magnesium oxide and other volatile impurities present in the original powder mixture of ingredients. For loads of 10 to 75 pounds powder a treatment period of between 1 and 3 to 5 hours has been found satisfactory. In general, it is desirable to employ as the initial ingredient a grade of amorphous boron which contains the smallest amount of non-volatile impurities. It is very important that the iron impurities be less than 0.2% of the original boron of the mixture and that it should be substantially free from silicon carbide and other silicon impurities, it being desirable that the final powder produced shall contain 96.5 to plus 99% of the primary desired ingredients, to-wit, zirconium, boron and carbon in amounts corresponding to zirconium diboride ZrB with 2 to 7% boron, wherein the boron contains 4 to 33 atomic percent carbon in solid solution therewith.

Upon completion of the heating cycle at 1800 C. carried on in the manner described above, the crucible with its load is removed from the high frequency field and subjected to cooling for a time depending on the magnitude of the load. A hydrogen atmosphere should be maintained in the space in which the treated powder is cooled for at least 20 hours subsequent to completion of the heating cycle. In the example here considered, good results are obtained by passing the hydrogen at a rate of 5 cubic feet to 15 cubic feet per hour during the first 24 hours from the completion of the heating cycle and thereafter cooling with a flow of at about 5 cubic feet per hour. Excess hydrogen flow through the crucible during the cooling period will result in formation of water vapor at the burn-out region and will tend to contaminate the cooled powder charge. If the powder body is prepared without the addition of carbon to the original powder mixture charge of zirconium and boron, the hydrogen may be shut ofi at the completion of the 1800 C. heating cycle described above, and a graphite block is placed over the exhaust. The powder charge in the crucible will then cool within a carbonaceous atmosphere generated by the graphite crucible and the excess boron of the charge will combine with the carbon of the carbonaceous atmosphere within the crucible into boron containing 4 to 33 atomic percent carbon in solid solution.

At the end of the cooling cycle, the powder charge in the crucible will form a lumpy semi-powdery body which is readily removed from the graphite crucible and pulverized by a simple crushing procedure. Iron or steel jaws or like crushers should not be used for this pulverization step since the powder body should be protected against very harmful contamination with iron. The crushed powder then is subjected to further comminution into a fine powder of a particle size such as between 2 and 8 microns average particle size. A gaseous-type vortex pulverizing equipment of the type described hereinbefore has been found satisfactory for this purpose. By way of example, on chemical analysis, a body produced in the manner described above contained about zirconium, 23% boron and 1 to 1.2% carbon, the body consisting principally of zirconium diboride ZrB with 4% of excess boron containing in solid solution an excess of 4 atomic percent carbon. In producing zirconium diboride ZrB powder with the desired excess of 2 to 7% carbon-containing boron, approximately 2% boron is usually lost through oxidation and this factor should be taken into account and compensated for in proportioning the original mixture of the ingredients of the zirconium and boron ingredients which are subjected to the treatment described above. Thus, in producing a powder body of ZrB containing 5% of carbon-containing boron in excess of the zirconium diboride ZrB the original mixture of the powder ingredients subjected to the treatment described above, should be calculated to contain about 2 to 8% excess boron over the amount required tZoByield the desired proportion of zirconium diboride In producing the powder mixture of ZrB with 2 to 7% of carbon containing excess boron, it is important that the mixture shall not contain any uncombined carbon in excess of about 1.5% and this applies also to the cemented bodies made of such powder mixtures. If the cemented zirconium diboride ZrB material of the invention containsin addition to the excess of 2 to 7% boron having in solid solution therein 4 to 33 atomic percent carbon more than 1.5 uncombined or free carbon, the strength of the material is substantially reduced, and for best results, the uncombined carbon should be less than 0.5% of the material, and such material may be readily produced by the processes herein described.

Production of cemented zirconium diboride bodies As explained above, shaped cemented bodies of zirconium diboride ZrB particles bound with 2 to 7% of excess boron containing carbon in solid solution, may be produced by the use of powder metallurgy or ceramic techniques. These techniques include hot pressing, cold pressing and sintering, extrusion and sintering, slip casting and sintering, and any combination of these techniques with intermediate presintering and machining operations.

Because of the unusually high stability of zirconium diboride ZrB mixed with 2 to 7% of carbon containing excess boron this powder body will not be detrimentally afiected by contact with carbon when forming it into the desired shape by hot pressing in a graphite die. In such hot pressing procedure, the die may be heated either by direct current conduction, or by induction heating. As explained above, hot pressing in graphite die may be carried on with a pressure of about 3 to 4 t.s.i. If the powder body is hot pressed in a die formed of cemented zirconium diboride ZrB bonded with 2 to 7% of carbon-containing boron, pressures of 10 to 12 t.s.i. and higher may be applied to the die. In general, pressing with pressures of at least about 2 t.s.i. at temperatures of at least about 2000 C. are

required for producing bodies of the desirable high by resistance heating ZrB bonded with 9. ave a slightly lower temperature than the heated powder -mixture.?".;.f; I

, vwaypf example-the sraphitedi is heated by inducpa iver -shon eriquwhen producing rocket nozzles" p raining zirconium diboride with an excess of 5% carboncontaining boron which is subjected to a pressure of 3 .s.i. within the graphite die in a very short period.

and corrosion-resistance at a I A similar zirconium diboride powder mixture was used formaking fuel nozzles by pressing the powder body hydrostatically in a rubber die within a pressure of to v 25 t.s.i. The green compact was then presintered at an elevated temperature between l200 and 1500" C. The presinteredshapcd compact was thereafter further shaped and machined to size with ordinary machine shop equipment, the compact having been linearly oversized by about 16% to compensate for shrinkage in the final sintering operation. After shaping, the presintered compact was subjected to the final sintering at a temperature between 2200 and 2700 0., thereby giving it ultimate density and strength.

Cemented bodies produced by hot pressing in the manner described above and consisting of zirconium diboride of excess boron containing 4 to 33 atomic percent carbon in solid solution had the folv lowing physical properties; p

l ,alt' renin oin 2500 c.4600 c.

Q ,Density 5.2 to 5.3 grams per cc.

' Hardness..

(cubic centimeter).

, Rockwell A, 87 to 89. I Weight gain inair at 11007 C. v a for 200 hours About-8 to milligrams per square cm. ,7 It had a stress-to-rupture life given inthe table below:

Under Hours At Temperature Stress, Until p.s.l. Rupturedv According to the invention the exterior surface of such shaped body which is to be exposed to corroding atmospheres at elevated temperatures of 700 C. and higher, is provided with an exterior surface stratum consisting of chromium oxide, dichromium boride Cr B, and an excess of chromium. The present invention is based on the discovery that when the exposed surface of a shaped body formed of particles of ZrB bonded with a 2 to 7% carbon-containingboron, is provided with a adhering surface stratum of'chromium about .001 to .020 inch in thickness, and thereafter treatedat an elevated tema perature between about 1200 and 2000 C., the boron in the underlying strata of the cemented zirconium diboride will diffuse into the adhering chromium; coating and form with the chromium a new compound of dichroboride, Cr B, with-an excess of chromium which 'v exhibits a much higher resistance to corrosion than the underlying vcemented material.

A satisfactory method of applying a chromium layer to theboron-bonded zirconium boride nozzle or other article is by electroplating. Standard plating methods may be employed. The following is an example of a satisfactory plating process. The article to be plated is first cleaned, for instance, in a 10% aqueous solution of sodium hydroxide solution, and then washed in hot wdghinglS pounds out of a powder body-conmeantime an. aqueous bath containing chromic" acid' (calculated as CrO and sulfate ion in the ratio of approxi-- mately 100:1 (for example, a bath containing 3'50 guof I CrO and 3.5 g. of sulfate ions per liter) is heated to about 60 C. Goodre sults are obtained by plating the" cleaned article'inthis bath-ata current density of 1 ampere persquare inch. The zirconium diboridebody v is made the anode" andchro'miunr the cathode. Such-- a" bath will 30.n1inutes. a coating of1001'5' V proximately'180 minutesa coating thickness of 001 inch.

may be employed. as

Too great a thickness of chromium plating may tend. to peel off. Good results are obtained with chromium platings of .002 to .004 inch thickness. Coatings of such thickness when subsequently convertedbythe'heat treatment to the surface stratum of chromium boride,plus chromium and chromium oxide have excellent adhering properties. The chromiumoxide may comprise as much as 30% of the said stratum; I

The plated productsuch as that produced by the above described process is then subjected to the heat treatment at between about 1200 C. to 2000 boron of the product dilfuses into the and/ or vice versa to convert a portion into the dichromium boride, CrB is carried out in an oxidizing or an inert atmosphere of helium, argon or in vacuum, for example." Since a certain amount of chromium oxide is desired in the final product a carbonaceous or hydrogen atmosphere arenot desirable since both carbon and hydrogen have a'tendency to reduce chromium oxides.

C., whereupon the chromium coating of said chromium Rocket nozzles made of the' cemented zirconium .diboride particles bonded with 2-7% carbon-containing.

boron containing 4-33 atomic percent of carbon as described in said cop'ending application Serial No. 363,043, now abandoned, but without the chromium boride coating of this invention, gave outstandingperformances when subjected to repeated firing tests. For example, tests were carried out at flame temperatures exceeding 2700 C. and chamber pressures of up (pounds per square inch gauge) with a variety of propellent mixtures of the oxidizing or reducing type and also with solid propellants. The coated product of thepresent invention equals the performance of the uncoated product in every respect and in addition has a considerably longer life than the uncoated product of said copending application. 1 I

It will be apparent to those skilled in the artthat the novel principles of the invention disclosed herein in connection with specific exemplifications thereof will suggest various other modifications and applications of the same.

It is accordingly desired that the present invention shall not be limited to the specific exemplifications shown or having a protective surface layer comprising dichromium chromium on at least boride Cr B, chromium oxide; and a portion of the surface area thereof.

2. A hard homogeneous body of high strength'and corrosion-resistance at high temperatures, at least about of said 'body consisting of cemented zirconium diboride, wherein particles of ZrB are bonded by a boron substance containing carbon in solid solution, said boron substance forming about 2 to 7% of said cemented zir onium diboride, said body being substantially free of produce in approximately. inch" thickness and in ap- The. heat treatment toI4000 p.s.i.g.

are bonded by aboron uncombined carbon over 1.5% of said body, said body having a protective surface layer comprising dichromium boride Cr B, chromium oxide, and chromium on at least a portion of the surface area thereof, the said protective surface layer being less than 0,004 thick.

References Cited in the file of this patent UNITED STATES PATENTS 1,803,189 Hoyt Apr. 28, 1931 1,853,369 Marshall Apr. 12, 1932 2,088,838 Cole et al Aug. 3, 1937 2,116,399 Marth May 3, 1938 Goetzel: The Iron Age (April 29, 1948), pp. 78-81. 10 Glaser: Trans. AIME (April 1952), Journal of Metals, pp. 391-396. v j 3 v Buckland et a1 Sept; 8, 1953 OTHER REFERENCES 

1. A HARD HOMOGENEOUS BODY OF HIGH STRENGTH AND CORROSION-RESISTANCE AT HIGH TEMPERATURES, AT LEAST ABOUT 95% OF SAID BODY CONSISTING OF CEMENTED ZIROCONIUM DIBORIDE, WHEREIN PARTICLES OF ZRB2 ARE BONDED BY A BORON SUBSTANCE CONTAINING CARBON IN SOLID SOLUTION, SAID BOROM SUBSTANCE FORMING ABOUT 2 TO 7% OF SAID CEMENTED ZIRCONIUM DIBORIDE, SAID BODY BEING SUBSTANTIALLY FREE OF UNCOMBINED CARBON OVER 1.5% OF SAID BODY, SAID BODY HAVING A PROTECTIVE SURFACE LAYER COMPRISING DICHROMIUM BORIDE CR2B, CHROMIUM OXIDE, AND CHROMIUM ON AT LEAST A PORTION OF THE SURFACE AREA THEREOF. 